In high speed optical telecommunication systems, one of the many technological challenges encountered is the chromatic dispersion induced in light signals propagating over long distances in optical media such as optical fibers. Chromatic dispersion causes light pulses to spread out as they travel along an optical fiber. Chromatic dispersion occurs because different spectral components in a light pulse travel at slightly different speeds. An optical pulse, including several different optical spectral components can therefore be broadened or distorted in shape after propagation through a sufficiently long distance in a dispersive optical medium. This dispersion effect can be undesirable and even adverse for certain applications such as optical communication systems where information is encoded, processed, and transmitted through optical pulses. As the pulses spread, they can overlap and interfere with each other, thereby impacting signal integrity and limiting the transmission bit rate, the transmission bandwidth, and other performance factors of the optical communication systems. This effect becomes even more pronounced at higher data rates, where subsequent light pulses are closer to each other.
Pulses at different wavelengths typically suffer different amounts of dispersion. The chromatic dispersion in standard single-mode optical fiber is nominally 17 ps/(nm·km) in the 1550 nm telecommunication window, but this value changes as a function of the wavelength: its value changes by about 2 ps/(nm·km) between 1530 nm and 1565 nm.
A known method for the correction of dispersion in optical fiber is the use of Fiber Bragg gratings (FBGs), a well-established technology for optical telecommunications. Basically, a Bragg grating allows light propagating into an optical fiber to be reflected back when its wavelength corresponds to the grating's Bragg wavelength, related to its period. A chirped FBG, in which the grating period varies along the fiber axis, represents a well-known solution for compensating the chromatic dispersion of an optical fiber link (F. Ouellette, “Dispersion Cancellation Using Linearly Chirped Bragg Grating Filters in Optical Waveguides”, Opt. Lett., 12, pp. 847-849, 1987). Such a grating compensates for the accumulated dispersion since the group delay varies as a function of the wavelength. An appropriate grating can be fabricated such that the wavelength dependence of its group delay is just the opposite of that of the fiber link.
In order to improve the versatility of dispersion compensating devices, it is known to provide means for changing the group delay induced by a Bragg grating, therefore tuning the dispersion characteristics of the device. This can for example be achieved by applying a temperature gradient to optical fiber, locally changing the temperature of the grating to affect its reflectivity characteristics. A practical and power efficient assembly for applying such a temperature gradient to a fiber Bragg grating is shown in assignee's U.S. Pat. No. 6,842,567 (LACHANCE et al). The temperature gradient is produced in a heat conductive element, with which the grating is in continuous thermal contact, by elements controlling the temperature of the ends of the heat conductive element, thereby applying the temperature gradient to the grating. A heat recirculation member may be provided to allow the recirculation of heat between the two ends of the heat conductive elongated element. Isolation from the surrounding environment may also be provided in order to decouple the desired temperature gradient from ambient temperature fluctuations. Different manners of using such an assembly are further disclosed in assignee's U.S. Pat. Nos. 6,937,793 (LELIÈVRE et al), 6,941,044 (PAINCHAUD et al).
Although prior art technologies enable the tuning of the dispersion characteristics of a dispersion compensator on a global or channel-per-channel basis, their specifications are provided with respect to a fixed grid of wavelength channels. This limitation has lead to the use of multiple components to cover the narrower channel spacing currently in use.
Recent driving forces in telecom systems markets have pushed manufacturers to lower their manufacturing cost and thus limit the number of configuration and wavelength dependent material they use and produce. One current problem they face is that commercially available tunable dispersion compensators have a fixed wavelength response due to the intrinsic nature of tunable solutions. Thus, current products are unable to fulfill the need for a 50 GHz grid spacing of true random central wavelength, or have limited tuning range on larger grid spacing.
There is therefore a need for a true colorless tunable dispersion compensator which maybe used in addressing any wavelength, not necessarily on a predefined pattern or grid, over a complete wavelength band of operation.